1. Field of the Invention
The present invention relates to a method for developing an electric latent image, or electrostatic latent image formed in the field of image forming process such as electrophotography and electrostatic recording method, by means of a liquid developer.
2. Description of the Prior Art
In the prior art of the image forming technique such as electrophotography and electrostatic recording method, there are already known various methods of developing a latent image formed on a carrier for electrostatic latent image such as a photosensitive member or electrostatic recording material by means of a developer in the form of liquid (hereinafter called liquid developer). Such liquid developing method comprises bringing a liquid developer, containing a dispersion of toner particles in an insulating carrier liquid having a volume resistivity of 10.sup.10 ohm.multidot.cm or above and a dielectric constant of 3 or below, for example paraffinic hydrocarbons, into contact with a surface holding an electrostatic latent image so that the toner particles in the carrier liquid are attracted by and adsorbed on the electrostatic latent image (hereinafter called electrostatic image), thereby rendering the electrostatic image visible. In such a kind of liquid developing method, the achievement of high-speed development has been an important object in recent years. In order to achieve the high-speed liquid development, it is necessary to supply a liquid developer of a high concentration to a surface holding an electrostatic image and to perform prompt recovery of excessive liquid developer with good efficiency. It is also important to obtain a developed image of satisfactory quality from which excessive liquid developer is sufficiently removed.
As a method substantially satisfying some of the above-mentioned requirements for such liquid development, there is known a method of employing an elastic member capable of holding liquid therein and provided with a liquid-permeable surface as a supply means for a liquid developer and conducting the development of an electrostatic image in a portion of the electrostatic image-holding surface where said elastic member is maintained in pressure contact therewith, by means of the liquid developer squeezed out of the elastic member. Such method is regarded superior to other liquid developing methods such as those employing a dipping tank for liquid developer and utilizing a jet of liquid developer in that the squeeze supply of liquid developer and suction removal of excessive liquid developer can be effected simultaneously in accordance with the pressure deformation of the elastic member. This elastic member can be formed into a rotary member in the shape of a roller or endless belt (hereinafter referred to collectively as elastic roller). As an example of such elastic roller there is disclosed, for example Japanese Patent Laid-Open No. 40336/1977, a roller of a structure in which a sponge layer is firmly fixed to a central shaft and its periphery is covered with a flexible net member. Such elastic roller is regarded considerably suitable and useful in the application for the development of an electrostatic image as it can be rotated while in pressure contact with a surface of a rigid member such as a photosensitive member or an insulating member to form a nip of a desired width and has functions of squeezing out liquid contained in the roller and absorbing the liquid thereinto. However, while the liquid developing method using an elastic roller is considered to have the above-mentioned advantages, some disadvantages as given below have been found from the developing result in such liquid developing method. That is, when the developing operation is repeated many times, the density of the developed image is gradually reduced and the image become blurred. Also in the copying operation for one sheet of paper, the image developed by a developing roller in the last copying cycle appears in another developed image, particularly in the black area of intermediate tone, obtained in the next copying cycle. In other words, the ghost image phenomenon takes place. Further, undeveloped area remains or lack of uniformity in image density is present in a portion of the visualized image.